1. Field of the Invention
This invention is directed to a system for detecting and initiating response, as required, to nuclear overpower conditions in reactor cores and for providing information useful for reactor control and surveillance purposes by using a minimum number of incore nuclear detectors grouped in redundant sets together with the required hardware and software for signal and data processing to continuously monitor and evaluate nuclear power level and nuclear power spatial distribution in the reactor core.
2. Prior Art
The fissionable material in a nuclear reactor is housed in pellet form in long metallic fuel rods which are mounted in spaced, parallel relationship in square frames to form elongated fuel assemblies containing for instance, two to three hundred rods. Typically, between one hundred and two hundred of these square, elongated fuel assemblies are massed together inside a reactor vessel to form a generally cylindrical reactor core in which the density of the neutrons produced by the fission reactions in the fuel pellets in successive generations is sufficient to sustain a chain reaction. The thermal energy produced by the chain reaction is absorbed by a reactor coolant, which circulates through the core, and is utilized externally to generate electric power in a turbine generator set.
In order to maintain the power of the reactor core at a given level, the neutron population must be controlled so that it remains constant in each successive generation. Excess neutrons produced by fission are removed from the fission process by introducing neutron absorbing materials into the core. Long term control of the reactivity in a pressurized water reactor (PWR) is achieved by disolving boron, a neutron absorber, in the reactor coolant. Short term control is effected by control rods containing neutron absorbant material which are inserted vertically into guide tubes or thimbles distributed across the fuel assemblies. A number of control rods, referred to as rodlets, are connected together at their remote ends by spiders to form control rod clusters which can be inserted into and withdrawn from the core simultaneously by a common drive mechanism.
Control of the reactivity of a reactor is complicated by the generation of xenon 135 which has a high cross-section of neutron absorption and therefore poisons the core. While some xenon is generated directly by fission, most of it is produced by decay of fission products and therefore, xenon production lags an increase in reactivity. Since some of the xenon is consumed directly by neutron absorption as well as by decay, a reduction in reactivity with the attendant reduction in neutron flux results in a large increase in xenon concentration. The localized effects on reactivity produced by the control rods affect the distribution of xenon, and therefore the ability to generate power in the near future, throughout the core.
Advanced PWR cores have two additional types of rods which are vertically inserted into and withdrawn from the reactor core. In any thermal fission reactor, the energy level of the neutrons produced by fission is generally too high to produce any significant fission reactions. However, the reactor coolant, ordinary water in the case of a PWR, in addition to serving as a heat transfer medium, also acts as a moderator which slows the fast neutrons down to a more suitable velocity for fission. The higher energy neutrons, however, are capable of transforming uranium 238 in the fuel into plutonium 239 which, in turn, can be used as a fission fuel. The conventional PWR does not utilize this phenomenon efficiently, but rather uses neutron absorbing material to absorb excess neutrons. Since excess reactivity is designed into the conventional PWR so that rated power may be maintained for a longer period of time as the core burns up, this results in an inefficient use of the nuclear fuel through high rates of absorption of neutrons early in the fuel cycle. The advanced PWR, on the other hand, utilizes the higher energy neutrons to produce plutonium which becomes available later in the fuel cycle for producing power. This is acheived by the use of water displacer rods made of neutron transparent material which are inserted into the core early in the fuel cycle. These rods, by displacing the moderator without absorbing neutrons, effect a spectral shift to higher energy neutrons which results in higher rates of production of plutonium. In addition to the control rods and water displacer rods, the advanced PWR also has gray rods which are between the control rods and water displacer rods in neutron absorption capacity. These gray rods are used in control of the neutron population and, like the water displacer rods, are either fully inserted or fully withdrawn. Like the control rods, the water displacer rods and gray rods are mounted on spiders to form rod clusters which are operated simultaneously with clusters symmetrically located in each quadrant of the core to avoid radial distortion of the power distribution.
In any nuclear reactor, it is important to monitor both the total power generated in the core and the spatial distribution of the power. Local hot spots can fault the fuel rod cladding, releasing fission products into the reactor coolant and thus compromising the first barrier for containment of the fissionable material. Local hot spots could result from improper sequencing of the rod clusters such as through failure of a drive mechanism. It could also arise from disengagement of a rod from a cluster. While an unplanned insertion of a control rod reduces the local reactivity, the control system, in an attempt to maintain a commanded total power, will increase the power in other parts of the core to a level which may approach design limits.
Many of the current systems for monitoring reactor power utilize detectors outside of the reactor vessel (excore detectors) to determine the total power as a function of the leakage flux. Certain assumptions are made from the individual readings on the excore detectors as to the spatial distribution of power within the core. In view of this, substantial margins must be built into the control system to assure that operating limits are not exceeded. It is also not always possible with such a detection system to determine the specific rod or rod cluster which is the source of such trouble.
It is common in commercial reactors to periodically insert movable neutron detectors into thimbles in the reactor core to map the neutron flux density. The movable incore detectors have also been used to calibrate the excore detector system. However, these are not protection grade and, for the most part, are only used periodically.
It is also common to group the detectors in independant channel sets to provide redundancy for the reactor protection system. Typically, an overpower indication on two out of four of the channel sets, or on two out of three or one out of two, if one or two channel sets respectively are out of service, initiates a reactor trip.
Some reactor power detection systems have used fixed incore detectors. In the system described in U.S. Pat. No. 3,565,760, the reactor core is divided into four quadrants so that an incore detector located at a given position in one quadrant provides a signal that is expected to be representative of the corresponding positions in all four quadrants. Detectors are positioned in a different radial and/or angular position in each quadrant so that each detector is in a unique position with respect to the symmetry of the core, and a sufficient number of the detectors is provided to furnish complete representative monitoring of the core. The detector outputs are each compared with a limit value to determine an overpower condition adjacent the detector; however, if a local hot spot exists in the symmetrical location where there is no detector it will go undetected. The outputs from the detectors are also grouped into several average power circuits with each group having detectors from representative locations across the core so that there are several independent signals representative of the core total power. The individual detector overpower signals and the total power signals are used in a plant protection system which trips the reactor if preset operating limits are exceeded.
The present state of the art leaves unfulfilled a need for an accurate, reliable on-line system for generating signals representative of the total power generated by a reactor core and the spatial distribution of that power. In particular, there is no available system which can accurately indicate a specific control rod cluster or water displacer rod cluster which is out of position by use of fixed incore detector strings in less than half the fuel assemblies especially with one or more groups of detectors out of service. The prior art systems also are not capable of pinpointing the location of a hot spot in the core down to a specific fuel rod location.